import numpy as np
import cv2 as cv

# Create a white image
img = np.ones((512,512,3), np.uint8)*255

# Draw a diagonal blue line with thickness of 5 px
startPt = (100,0)
endPt   = (img.shape[1]-1, img.shape[0]-1)
color   = (255,0,0)
thinknessPx = 5
cv.line(img, startPt,endPt, color,thinknessPx)

cv.imshow("img", img)
cv.waitKey(0)
cv.destroyAllWindows()